Percarboxylic acids (peroxycarboxylic acids) such as in particular peracetic acid and performic acid are highly effective microbiocides which are used in a very wide range of different water systems for controlling microorganisms, reference being made, by way of example, to WO 97/08100 and EP 0 688 302. The percarboxylic acids mentioned are especially effective in the form of a so-called aqueous equilibrium percarboxylic acid solution which consists substantially of a percarboxylic acid, the carboxylic acid from which it is derived, hydrogen peroxide and water and may also contain a mineral acid catalyst and conventional active oxygen stabilizers in effective amounts.
In order to satisfy specific application-oriented requirements, additives are added to the percarboxylic acid solutions mentioned, such as wetting agents, emulsifiers, corrosion inhibitors and buffers. Microbiocidal compositions in accordance with WO 93/10088 or WO 94/14321 are based on an equilibrium percarboxylic acid and non-ionic surfactants. A similar composition, but with an anionic surfactant selected from the group alkylbenzene sulfonates, alkyl sulfates and alkane sulfonates instead of the non-ionic surfactant, is disclosed in FR 2 321 301 and DE 26 16 049, wherein the composition in the last-named document also contains a phosphonic acid. A stabilised aqueous peracetic acid solution with increased antimicrobial effect and a reduced corrosive effect contains, according to DD 96833, a buffering agent, wetting agents or emulsifiers, such as alkali metal salts of sulfonated fatty acids, fatty alcohol sulfonates, alkane sulfonates and alkylaryl sulfonates, and organic sequestering agents such as dipicolinic acid and ethylenediamine tetraacetic acid. Finally, thickened percarboxylic acid solutions are known, such as those in accordance with EP 0 421 974 B1, which contain a cross-linked acrylic acid polymer and also a sequestering agent from the group of phosphonic acids and nitrogen-containing carboxylic acids.
In water-circulating systems, there is often the need to simultaneously avoid or reduce problems due to the presence of microorganisms and the production of solid coatings of hardness-producing components. Hitherto, a hardness stabilizer, in particular one from the group of phosphonocarboxylic acids, and an equilibrium peracetic acid have been added, separately, to the circulating system for this purpose. This addition process is costly. Although a solution of an equilibrium peracetic acid and a phosphonocarboxylic acid exhibit the required microbiocidal and hardness-stabilizing effect, the storage stability is inadequate.